Longest Increasing Subsequence (LIS): the O(n log n) patience-sorting trick.
LIS has an obvious O(n²) DP and a clever O(n log n) solution that catches people off guard. Interviewers look for the patience-sorting/binary-search approach that maintains 'tails'. Both appear below.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
LIS has an obvious O(n²) DP and a clever O(n log n) solution that catches people off guard. Interviewers look for the patience-sorting/binary-search approach that maintains 'tails'. Both appear below.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
